2 Fig. 2.1 shows a cross-section through a soil profile in a tropical grassland. (a) Name the soil horizons labelled W, X, Y and Z on the diagram. [4] (b) E...

Labelled answer diagram:

(a) Names of the four soil horizons shown in the cross-section (Fig. 2.1):

(b) Horizon X (topsoil) is darker than the horizons below because it contains the highest concentration of humus - decomposed organic matter from plant roots, fallen leaves, and animal remains. Humus is dark brown to black in colour. Deeper horizons contain progressively less organic matter (most decomposition and biological activity occur near the surface), so they are lighter in colour, dominated by mineral tones. [2]

(c) Minerals move from horizon X to horizon Y through the process of leaching, which occurs in three stages:

1. Dissolution - rainwater percolating through the topsoil dissolves soluble minerals and ions (such as iron, aluminium, calcium, and carbonates) from the soil particles and organic matter. [1]
2. Downward transport (eluviation) - the mineral-laden water moves downward through the soil profile under the force of gravity, carrying dissolved and suspended materials out of horizon X. [1]
3. Deposition (illuviation) - as the water reaches horizon Y (the subsoil), conditions change (e.g. lower organic acid content, different pH, finer pore structure) and the dissolved minerals precipitate or are adsorbed onto soil particles, accumulating in this layer. This is why the B horizon is sometimes called the "zone of accumulation." [1]

(d) Two ways parent rock (horizon Z) contributes to soil formation above it:

1. Physical weathering - temperature changes, frost action, and root growth fracture the rock into progressively smaller fragments that become the mineral skeleton (sand, silt, clay particles) of the soil. [1]
2. Chemical weathering - water and acids react with the rock minerals, breaking down their crystal structures and releasing nutrient ions (potassium, calcium, magnesium, phosphorus) into the developing soil. [1]

(e) Ploughing is normally limited to horizon X (topsoil) because:

1. Topsoil is workable - it has a loose, crumbly structure with relatively high organic matter content, making it soft enough for plough blades to cut through and turn over. [1]
2. Subsoil and bedrock resist tillage - horizon Y is denser and more compacted (often with a higher clay content from illuviation), and horizon Z is solid or semi-solid rock. Plough blades cannot penetrate these layers without excessive force and equipment damage. [1]

(f) Two methods to increase the depth of horizon X:

1. Regular addition of organic matter - applying compost, farmyard manure, or crop residues builds up the humus content of the topsoil, physically deepening the A horizon over time. [1]
2. Growing and incorporating green manures or cover crops - deep-rooted legumes and grasses are grown and then ploughed in, adding organic matter throughout the root zone and extending biological activity deeper into the profile. [1]